Track definition merging on the canonical declaration even when local

Track definition merging on the canonical declaration even when local
submodule visibility is disabled.

Attempting to pick a specific declaration to make visible when the
module containing the merged declaration becomes visible is error-prone,
as we don't yet know which declaration we'll choose to be the definition
when we are informed of the merging.

This reinstates r342019, reverted in r342020. The regression previously
observed after this commit was fixed in r342096.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@342097 91177308-0d34-0410-b5e6-96231b3b80d8